[QUOTE=Loach]
Eamon Walker who I know mostly from the HBO series Oz. A flawless accent in my opinion.
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I was gonna mention Walker! I saw him on a UK show and I was like, “Is that Said? No way!”
The bloke who plays Carcetti on The Wire - Aidan somebody - is at times good, as cited upthread, but this past season he lost it and became this nasally Irish-American-sounding accent at times.
I think Hugh Laurie is great. Listen to him play an American from his sketch days on A Bit of Fry and Laurie - cringeworthy there, quite good now.
I also think Rachel Griffiths did great on Six Feet Under.
The guys who did the voices on Spitting Image in the 80s - including Harry Enfield - did great accents. Enfield is so good, most people didn’t know his annoying “Dr. Angus” character from the Burger King commercial wasn’t American. His sketches have very good, middle-American accents. His last series with Paul Whitehouse was alternately brilliant (his accent) and hideous (Whitehouse’s).
Bad accents:
Tracey Ullman can definitely go overboard. Most of the time she’s good, but other times not so much. Her Three Of A Kind costar, Lenny Henry, does a funny (but pretty bad) American accent.
Daphne from Frasier is horrible. I really hated her guest spot on Seinfeld. I was gobsmacked to learn she was a Brit.
I’ve already mentioned Whitehouse. Americans won’t know him (he was a major player on The Fast Show/Brilliant!).
Americans have totally tin ears for non-American accents, which I find annoying. I have no problem understanding most Commonwealthers - Jamaicans, Aussies, and so on - but I remember being amused when Oasis were big in America, they had Noel Gallagher’s interviews with subtitles. Liam’s a bit mushmouthed but Noel is quite understandable.